Abstract (inglese)

In the framework of magnetically confined fusion plasmas, achieving a comprehensive understanding of Plasma-Wall Interaction (PWI) is fundamental to attain the improved confinement regimes, needed to produce fusion energy. A major issue is the lack of plasma density control observed in devices equipped with a carbon first wall. Carbon Plasma First Components (PFCs) offer the advantage of withstanding high power loads that can reach tens of MW/m2, but present the drawback of high retention of hydrogenic fuel particles. Fuel recycling, that is the churning action of these retained fuel particles between the hotter fusion plasma edge and the colder plasma facing first wall surfaces, presents often a factor exceeding 1 and makes the task of density control extremely difficult.

Wall conditioning techniques have proven to be a great tool to overcome the carbon limiting performance factor. In the DIII-D tokamak [1] very high confinement regimes (VH-modes) were attained by solely applying a thin boron film on the plasma facing surfaces; earlier in the Tokamak Fusion Test Reactor (TFTR) [2] significant improvements in ‘supershot’ performances leading to a near doubling of the fusion power output were observed for the first time by injecting as small as a few milligrams of lithium during plasma discharges. The implementation and the optimization of wall conditioning techniques, together with the study of new Plasma Facing Materials (PFMs) is therefore clearly fundamental.

As many other fusion devices, RFX-mod is equipped with a polycrystalline graphite first wall. Improved confinement regimes were found during high current operation, the so called Single Helical-Axis states (SHAx) [3], characterized by a helical deformation of the plasma column and the formation of an internal transport barrier. However, the full attainment of such improved confinement regimes is hindered by a lack of density control. To address this issue, a set of wall conditioning techniques has been implemented, including Helium Glow Discharge Cleaning (HeGDC), boron wall conditioning by B2H6 + He glow discharge and recently lithium wall-coating by means of pellet injection and lithium evaporation. However the beneficial effects of these techniques on RFX-mod plasmas are limited due to a lack of optimization and understanding of the fundamental mechanisms that govern the enhanced PWI observed upon the conditioning of the first wall surfaces.

In this thesis, the optimization and further implementation of the wall conditioning techniques has been undertaken. First, HeGDC has been characterized in terms of the glow discharge experimental parameters. Next, its efficiency combined with He plasma power discharges is examined in terms of H wall-depleting. On the other hand, a set of surface science techniques including SIMS, XPS, AES, EDS/X, SEM and RBS has been set up to investigate by ex-situ post-mortem analyses the underlining physical and chemical properties of boron and lithium thin film deposition after boron and lithium wall conditioning. Regarding the GDC optimization the main result is that a strong toroidal asymmetry was found and confirmed by measurements of the ion flux to the wall. Moreover, the preliminary analysis of intershot HeGDC wall cleaning strategy was found promising. With respect to the boronization optimization, evidence of a two-step boron growth with increasing glow discharge power was found. As for lithium wall conditioning, a correlation among the injected lithium dose and the chemical fraction of lithium carbonate was found.

Ultimately, a transition towards a metallic first wall in RFX-mod offering a low recycling surface has been recently considered as an alternative solution-path to overcome the graphite first wall drawbacks. Tungsten (W) coatings on graphite samples have been elaborated using two different Physical Vapor Deposition (PVD) techniques: High Power Impulse Magnetron Sputtering (HiPIMS) technique developed at the CNR-IENI laboratory in Padova, Italy, and Combined Magnetron Sputtering and Ion Implantation (CMSII) technology developed at MEdC-Romanian Euratom Association. The W-coatings were preliminary characterized by Scanning Electron Microscopy (SEM) and tested with adherence tests before exposing them to RFX-mod plasmas. Plasma exposure was made in experimental sessions of about 15 discharges, during which the interaction with the plasma has been locally enhanced by the active control system of the magnetic boundary in order to simulate the conditions of the PWI events. The CMSII technique, that gave the minor faults at the first stage, was then tested on full scale dimension by replacing 4 actual RFX-mod carbon tiles with W-coated ones. The tiles have been exposed to three months of normal RFX-mod operation over a large range of operational conditions, after which large damaged areas have been observed. The size of these damages and particularly the removal of the coating in some areas and its melting may discard this solution-path towards such a metallic PFC transition in RFX-mod. Thicker 100-200 µm W-coatings, that are still compatible with wall weight tolerance of RFX-mod mechanical structure, may offer a better solution for the machine upgrade.

3115. doi:10.1016/j.jnucmat.2009.01.167. Proceedings of the 18th International Conference on Plasma-Surface Interactions in Controlled Fusion Device Proceedings of the 18th International Conference on Plasma-Surface Interactions in Controlled Fusion Device.
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